Fix compilation of image.c on MS-Windows
This is a followup to last change in image.c. * src/image.c (rsvg_handle_get_intrinsic_dimensions): Define to call fn_rsvg_handle_get_intrinsic_dimensions. (svg_css_length_to_pixels): Compile only for librsvg >= 2.46.0, as RsvgLength type was not defined before.